class Solution {
    //https://leetcode.com/problems/palindrome-pairs/discuss/79195/O(n-*-k2)-java-solution-with-Trie-structure
    private static class TrieNode {
        TrieNode[] next;
        int index;
        List list;
            
        TrieNode() {
            next = new TrieNode[26];
            index = -1;
            list = new ArrayList<>();
        }
    }
        
    public List> palindromePairs(String[] words) {
        List> res = new ArrayList<>();

        TrieNode root = new TrieNode();
            
        for (int i = 0; i < words.length; i++) {
            addWord(root, words[i], i);
        }
            
        for (int i = 0; i < words.length; i++) {
            search(words, i, root, res);
        }
        
        return res;
    }
        
    private void addWord(TrieNode root, String word, int index) {
        for (int i = word.length() - 1; i >= 0; i--) {
            int j = word.charAt(i) - 'a';
                    
            if (root.next[j] == null) {
                root.next[j] = new TrieNode();
            }
                    
            if (isPalindrome(word, 0, i)) {
                root.list.add(index);
            }
                    
            root = root.next[j];
        }
            
        root.list.add(index);
        root.index = index;
    }
        
    private void search(String[] words, int i, TrieNode root, List> res) {
        for (int j = 0; j < words[i].length(); j++) {   
            if (root.index >= 0 && root.index != i && isPalindrome(words[i], j, words[i].length() - 1)) {
                res.add(Arrays.asList(i, root.index));
            }
                
            root = root.next[words[i].charAt(j) - 'a'];
            if (root == null) return;
        }
            
        for (int j : root.list) {
            if (i == j) continue;
            res.add(Arrays.asList(i, j));
        }
    }
        
    private boolean isPalindrome(String word, int i, int j) {
        while (i < j) {
            if (word.charAt(i++) != word.charAt(j--)) return false;
        }
            
        return true;
    }
}